49°
METALLURGY OF CAST IRON.
bar. This is caused by the lower half being- cooled more quickly than the upper half. This gives in the lower half of the bar, in a sense, more combined than graphite carbon, which results with iron not " white " in causing the " lower " half to be of greater strength than the upper half. But the degree to which this is affected in flat-poured bars is largely controlled by the difference in the "temper" of the sand, hardness of ramming, degree of fluidity, speed of pouring, and the quality of iron used. Since these conditions can-
FIG. ICQ.
not be always the same, results in testing flat cast bars are erratic. That one side of a flat cast bar will always be in line of giving more strength than another, is understood when we take into consideration, with the above, the fact that in testing for transverse strength, we subject the under side of the bar to an extension or tensile strain, and the upper side to one of compression or crushing. If we have the densest or highest combined carbon side of a bar to resist the extension or tensile strain, it is reasonable tors, the}-should always be moulded in the same flask, poured from the same ladle and from the same gate. To prove my position on this question, I would lirst rail attention to conditions which can be found by any who arc sufficiently interested to experiment in this line. In Pig;. 109, next pag'e, is shown a side elevation of a bar resting* on pointed supports A B, 12 indies apart, the distance which the author used in his experiments. The point of load is shown at, I >, The position of the bar is the same as when east or lying" in its mould. In exam in« ing such a bar it will be found that the metal at the lower side or shell 1C E is generally denser, or of a closer gram, than that composing' the upper half of the Chapter I,XX,, par,*-*; :;y.| in r,.s.j. and attacked the usual formulae for loaded beams asound burs..........................*fi$7 "